The goal of this proposal is to investigate the relationship between Dkk/LRP interaction and bone formation. Both human and mouse genetic evidence shows that loss of function LRP5 alleles cause osteoporosis, while putative gain of function alleles are associated with high bone mass (HBM). We showed that differentiating osteoblasts produce an autocrine canonical Wnt, which can stimulate osteoblast proliferation and differentiation and be blocked by pretreatment of Dkk. In addition, we found that HBM G171V mutation may increase Wnt signaling by preventing the inhibition of LRP5 by paracrine Dkk. Together with our finding that Dkk2 may be involved in terminal osteoblast differentiation, we hypothesize that moderate reduction in Dkk-mediated antagonism to LRP5-mediated signaling may increase bone mass, while severe reduction may cause a loss of bone formation. In this application, we will test this hypothesis by examining the effects of reduction in Dkk-mediated antagonism on bone formation in mice. Specifically, we will: 1) Determine if attenuation of Dkk-mediated antagonism by reducing Dkk1 or Dkk2 expression leads to an increase in bone formation in mice. Mouse models that express Dkk at varying levels will be created using a number of transgenic approaches and examined for the relationship between Dkk expression level and bone mass. 2) Determine if direct disruption of Dkk-LRP5 interaction increases bone mass. Transgenic mice expressing LRP5 mutants that contain mutations in the Dkk-binding surface will be created and examined for bone phenotypes. 3) Identify small molecule compounds that disrupt the LRP-Dkk interaction and Mesd- LRP interaction and determine their effects on osteogenesis. We have used a highly innovated approach that combines structural biology, in silico screen, and biological assays to successfully identify two small molecule compounds that may bind to the Dkk binding surface of LRP5. These compounds disrupt the binding of Dkkl to LRP5 and inhibit Dkk-mediated antagonism. Importantly, they increase osteogenesis in culture models. In this proposal, we will identify more efficient compounds and also carry out tests using in vivo models.